1. Technical Field
This invention relates to a bolometric detector and a manufacturing process for this detector.
It is particularly applicable to infrared imagery.
2. State of Prior Art
Infrared radiation detectors are already known, most of which are designed in matrix form and are capable of operating at ambient temperature, in other words without cooling, unlike devices called xe2x80x9cquantum detectorsxe2x80x9d.
These uncooled detectors usually use the variation of a property of an appropriate material as a function of the temperature, at about 300 K. For bolometric detectors, this property is the resistivity.
This type of uncooled detector usually combines (a) means of absorption of infrared radiation and conversion of this radiation into heat, (b) thermal insulation means for the detector, so that the detector can warm up, (c) thermometry means which, in the case of a bolometric detector, use a resistive element and (d) means of reading the electrical signals output by the thermometry means.
Detectors to be used for infrared imagery are made in the form of a matrix of elementary detectors, this matrix having one or two dimensions, on a substrate that is usually made of silicon and comprises means of electrical stimulation of elementary detectors by bias of these detectors, and means of detection and preprocessing of electrical signals output by these elementary detectors. These detection and pre-processing means are formed on the substrate and form a read circuit.
Monolithic integration of detectors in the corresponding read circuit is useful from the point of view of manufacturing costs. However, it is also possible to hybridize a matrix of elementary detectors onto this read circuit.
A detector comprising a matrix of elementary detectors and the associated read circuit is usually located in a housing and is connected to the outside medium by conventional techniques. In the housing, the pressure is reduced to limit temperature losses and this housing is equipped with a window transparent to infrared radiation to be detected.
To observe a scene through this detector, the scene is projected through appropriate optics onto the matrix of elementary detectors and electrical stimuli are applied at a constant rate through the read circuit (also provided for this purpose), to each elementary detector or to each row of these detectors in order to obtain an electrical signal forming the image of the temperature reached by each elementary detector. This signal is processed more or less extensively by the read circuit and then possibly by an electronic device placed outside the housing in order to generate an image of the observed scene.
The performances and the cost of uncooled bolometric detectors depend essentially (1) on control over the production and integration of the very high performance bolometric materials, (2) control of the manufacture of microbridges which are light and fragile structures capable of thermally isolating elementary detectors in the read circuit, (3) the construction quality of these detectors and various correction functions that are used in the read circuit and in other peripheral devices and (4) control of techniques and the cost of packaging in a housing.
This invention is related to point (1) above, and can be used to obtain very high performance bolometric detectors using relatively simple techniques.
A bolometric detector based generally on amorphous silicon is described in the following document:
[1] U.S. Pat. No. 5,021,663 A (L. J. Hornbeck).
This known detector has many disadvantages and particularly (1) a large number of different layers is necessary to manufacture it, particularly two parallel electrically conducting layers located on each face of the layer of the bolometric material in the detector, and (2) it is not possible to adjust the electrical resistance of this detector on a given surface, except by modifying the resistivity or the thickness of the bolometric material.
Another bolometric detector is described in the following document:
[2] U.S. Pat. No. 5,367,167 A (W. F. Keenan).
This other known detector comprises two coplanar electrodes located on the same face of the layer of bolometric material (usually made of amorphous silicon) and an electrically conducting layer that is located on the other face of this layer of bolometric material. The function of this conducting layer is to absorb the infrared radiation that is to be detected and it must be separated from the body of the detector by an electrically insulating layer.
The detector defined in document [2] corrects some disadvantages of the detector known according to document [1], but it still has disadvantages (1) and (2) mentioned above.
Furthermore, a bolometric detector with microbridges is described in the following document, which should be referred to:
[3] FR 2752299 A corresponding to EP 0828145 A and the American patent application Ser. No. 08/905059, Aug. 1, 1997 (M. Vilain and J. J. Yon).
The detector known as described in this document [3] does not have the disadvantages (1) and (2) mentioned above. It uses a single electrically conducting layer that is directly in contact with the bolometric material. In this case, the electrodes are formed from this single layer which also performs an optical absorption function.
The structure of an elementary detector according to the information given in document [3] is simpler than the structure of a detector made in accordance with document [1] or [2], and the electrical resistance of this detector can be adjusted within a wide resistance range for a given available detector area and for a given thickness and conductivity of the bolometric material used.
Therefore, the resistance can be optimized to obtain good coupling with the read circuit with limited constraints on the optimization of other detector parameters, namely (1) the thickness of the bolometric material on which the thermal capacity Cth and the thermal insulation Rth of the detector partly depend, and therefore the thermal time constant of this detector equal to the product of Cth and Rth and (2) the resistivity of the bolometric material that controls the TCR coefficient of the bolometric material, in other words the logarithmic derivative of the electrical resistance of this material with respect to the temperature.
However, note that the design of the electrically conducting elements (control electrodes and possibly electrically floating electrodes) must obey specific criteria in order to optimize the efficiency of the optical absorption function and finally the performances of the bolometric detector; in particular, the xe2x80x9cpitchxe2x80x9d of this design, in other words the sum of the values 1 and e in the example in FIG. 2 that will be described later, must be between 5 xcexcm and 10 xcexcm for optimized detection of infrared radiation with wavelengths between 8 xcexcm and 14 xcexcm. Furthermore, it is preferable that the values of 1 and e are similar. A reduction of e will cause an increase in the electrical noise level and an increase in e will tend to reduce this noise level but the resulting loss of optical absorption is preponderant.
The design of the bolometric detector described in document [3] is valid from the point of view of the compromise between performances and cost due to the simplicity of the structure of this detector which gives high efficiencies, but an attempt is made to significantly improve these performances by improving this structure.
Another purpose of this invention is to achieve this improvement and proposes a bolometric detector with excellent performances regardless of the bolometric material used, for example amorphous silicon, or a comparable material which naturally has a high level of low frequency noise, while keeping most of the advantages of the bolometric detector described in document [3].
In this known detector, geometric constraints are imposed on the surfaces of the electrodes (width and pitch).
In this invention, these constraints are overcome by means of additional electrical insulation in order to dissociate the constraints imposed in space between the electrodes, from constraints on the absorption of incident radiation. This thus increases the performances of the detector by a noise reduction and an increase in optical absorption.
More precisely, the purpose of this invention is a bolometric detector comprising a layer of bolometric material (material for which the resistivity varies as a function of the temperature) and at least two electrodes formed facing the same face of this layer of bolometric material and starting from the same layer of electrically conducting material, this detector being characterized in that each of the two electrodes comprises at least a first area and at least a second area, in that the second areas belonging to the two electrodes respectively are electrically isolated from each other and electrically isolated from the layer of bolometric material, and in that the first areas belonging to the two electrodes are at a spacing from each other and are in electrical contact with this layer of bolometric material.
According to a first particular embodiment of the device according to the invention, the second areas are electrically isolated from the layer of bolometric material by a layer of electrically insulating material.
For example, this electrically insulating material may be chosen from the group comprising silicon nitride and silica.
According to a second particular embodiment of the device according to the invention, the second areas are electrically isolated from the layer of bolometric material by a space in which there is no material.
Preferably, the bolometric material is chosen in the group comprising amorphous silicon, vanadium oxides, amorphous SiGe and SixGeyCz where xxe2x89xa70,yxe2x89xa70,zxe2x89xa70 and x+y+z=1.
For example, the electrically conducting material may be titanium nitride.
According to a preferred embodiment of the invention, the bolometric detector has a microbridge structure.
This invention also relates to a bolometric detector with a matrix structure comprising at least two detectors conform with the invention.
This invention also relates to a manufacturing process for the bolometric detector defined in the invention, in which a first auxiliary sacrificial layer is formed on a substrate, the layer of bolometric material and the electrodes are formed on this first auxiliary layer, by electrically isolating the second areas of electrodes from the layer of bolometric material and eliminating the first auxiliary layer.
According to a first particular embodiment of the process according to the invention, the layer of electrically insulating material is also formed to separate the second areas from the layer of bolometric material.
According to a second particular embodiment, a second auxiliary sacrificial layer is also formed to separate the second areas from the layer of bolometric material, and this second auxiliary layer is also eliminated.